1. Field of the Invention
The present invention relates to an electrode suitable for use in a light-emitting tube of a metal vapor discharge lamp, and to a method for manufacturing the same. Furthermore, the present invention also relates to a metal vapor discharge lamp.
2. Related Background Art
Recently, metal vapor discharge lamps have been developed that employ ceramic light-emitting tubes with superior heat resistance so as to achieve high color rendering properties and energy efficiencies, which increases the complexity of the manufacturing process.
The following will describe a conventional method for manufacturing electrodes for use in discharge lamps.
FIG. 9 is a schematic side cross-sectional view for explaining a configuration of a conventional method for manufacturing an electrode, in which two rod-type electrode parts are welded. In FIG. 9, 3a and 3b denote rod-type electrode parts to be welded, and 20a and 20b denote a pair of electrodes of a resistance welding machine. The electrode parts 3a and 3b are supported by the pair of electrodes 20a and 20b so as to be aligned with each other with the ends of the electrode parts 3a and 3b brought into contact. Forces F0 in upset welding are applied in directions so as to press the electrode parts 3a and 3b against each other via the pair of electrodes 20a and 20b, and current is caused to run through the electrode parts 3a and 3b via the electrode 20a and 20b. A heat generated by a resistance at an interface between the contact ends of the electrode parts 3a and 3b melts the contact ends, thereby bonding the same. Here, a high-purity argon gas is blown to the contact ends of the electrode parts 3a and 3b at all times.
Such a resistance welding method is effective in the case where both the electrode parts 3a and 3b are made of metals, but the method has a drawback in that the bonding is not achieved surely in the case where at least one of the electrode parts is made not of a metal but of a cermet. Since a cermet is a material obtained by sintering alumina and a metal, it has properties both of a ceramic and a metal. Therefore, it is difficult to melt the interface portions surely so as to bond the same, with only the aforementioned instantaneous heating by the resistance welding.
Furthermore, apart from the aforementioned resistance welding method, a method has been proposed in which the electrode parts 3a and 3b are supported with each other with their ends brought into contact, and in this state, the contact ends are irradiated with a laser beam such as a CO2 laser or a YAG laser. However, in the case of such a welding method with a laser beam, since the laser beam has a cross section of an approximately round shape, the projection of the laser beam on the contact ends causes heating irregularities to be generated in a circumferential direction. Hence, it is difficult to bond the border faces surely. Furthermore, since portions of the electrode parts other than the contact ends in a lengthwise direction of the electrode parts are heated as well, in the case where materials of the electrode parts contain tungsten, tungsten becomes brittle, which makes it impossible to secure a strength as an electrode.
The present invention is intended to solve the foregoing problems of the prior art, and it is an object of the present invention to provide an electrode manufacturing method that allows two electrode parts having different melting points, like those made of a metal and a cermet, to be bonded surely. Furthermore, another object of the present invention is to provide a discharge lamp employing an electrode manufactured by the foregoing manufacturing method. Furthermore, still another object of the present invention is to provide an electrode having a sufficient bonding strength, and a discharge lamp employing the electrode.
To achieve the foregoing object, an electrode manufacturing method of the present invention is a method for manufacturing an electrode by bringing an end of a first electrode part that is in a rod shape into contact with an end of a second electrode part that is in a rod shape and has a melting point higher than that of the first electrode part, and welding the same. The method includes the steps of arranging the first electrode part and the second electrode part on an upper side and on a lower side, respectively, with their lengthwise directions being aligned vertically and lineally, so that ends of the first and second electrode parts are brought into contact and pressed against each other, and subsequently welding the electrode parts by irradiating contact ends of the electrode parts or vicinities thereof with a laser beam. In this method, the laser beam has a cross section in a long narrow shape having a minor axis directed in a vertical direction and a major axis directed in a horizontal direction.
Furthermore, an electrode of the present invention includes a first electrode part that is in a rod shape and a second electrode part that is in a rod shape and has a smaller diameter than that of the first electrode part, with the first and second electrode parts being welded and integrated with each other in a state in which ends thereof are brought into contact. In the electrode, the first electrode part is made of a conductive cermet, the second electrode part is made of tungsten, and in a welded portion where the first and second electrode parts are welded, an alloy layer comprising molybdenum composing the conductive cermet of the first electrode part and tungsten of the second electrode part covers an end of the second electrode part.